Biological Considerations in Restorative Dentistry PDF

BIOLOGICAL CONSIDERATIONS IN RESTORATIVE DENTISTRY Intended Learning Outcomes: By the end of this chapter, the student will be able to: 1. Identify the aim of the biological bases of restorative dentistry. 2. Understand factors affecting the pulp response to restorative materials and procedures. “Do...

BIOLOGICAL CONSIDERATIONS IN RESTORATIVE DENTISTRY Intended Learning Outcomes: By the end of this chapter, the student will be able to: 1. Identify the aim of the biological bases of restorative dentistry. 2. Understand factors affecting the pulp response to restorative materials and procedures. “Do Not Harm” is a basic principal which should be followed by all members of the health professions. The biological bases of restorative dentistry aim to maintain: a. The health and vitality of the tooth structure to perform its formative nutritive, sensory and defensive functions. b. To keep the integrity of the investing structures i.e. the gingival and periodontium. I. TOOTH STRUCTURE A. ENAMEL: The hardest tissue in the body, forming the external tooth surface. It acts as a protective natural barrier for the vital dentin-pulp organ. Mature human enamel is highly mineralized 96% (by weight) with only 1% organic molecule and 3% water. The cementing substance between enamel rods is weaker than the rods, so: 1) During cutting with sharp cutting instrument, it is easily broken and split than the rods. 2) The rods are more resistant to dissolution by acid attack than the cementing substance during caries process and etching. B. DENTIN-PULP ORGAN: Dentin and pulp are considered as one organ because they are related to each other either: Embryologically: Both dentin and pulp are derived from dental papilla. Anatomically: Dentinal tubules contain the odontoblastic process of the odontoblast & the cell body is located at the pulp. Physiologically: Dentin is a protective tissue for pulp, while pulp provides nutrition to dentin. 1|Page Dentin: It is a calcified connective tissue composed of 70% inorganic material (hydroxyapatite), 20 % organic (mainly collagen) and 10% water (by weight). Dentinal tubules are the major channels for fluid diffusion across dentin. Pulp: All components of the dental pulp, including the cells, blood and lymph vessels, nerves, and the interstitial fluid, are important in the response to restorative procedures. Most of the pulp cells are considered to be of an undifferentiated type. II. SENSITIVITY OF DENTIN The most acceptable theory that explains dentin sensitivity is the hydro-dynamic theory: Hydrodynamic theory: The fluid in the dentinal tubules comes from the intercellular fluid of the pulp connective tissue. When this fluid moves in the tubule, direct mechanical deformation on the A-delta free nerve fibers within the tubule occurs and leads to depolarization of the A-delta fiber membrane and action potential leads to pain impulse. III. PRIMARY ATTACKING MECHANISMS Types of irritants: Bacterial Mechanical Chemical Osmotic Thermal Evaporative Galvanic 2|Page IV-Factors affecting the pulp response to restorative materials and procedures: 1. The degree of irritation and the reparative power of the pulp: Caries may be slow and less virulent with good pulpal condition and reparative potential, so that differentiation of the undifferentiated mesenchymal cells into odontoblast like cells takes place to lay down impermeable a- tubular (reparative) dentin. If caries rapidly progressing, a defensive mechanism that walls off the irritation by hyper-mineralization of the dentin underlying the lesion (dentin sclerosis). If more irritation occurs, the odontoblasts will degenerate completely to form dead tracts. The dead tract is more permeable to detrimental chemicals and toxins than vital dentin and it deprive the dentin of its elasticity. Pulpal inflammation is the worst-case scenario in case the defense mechanism could not overcome the strongly attacking insult leading to pulpitis or even periapical pathosis. 2. The condition of the pulpal floor: Microscopic examination of the pulpal floor of deep cavities (less than 1/2 mm remaining dentin thickness) reveals presence of multiple, small and non- hemorrhagic pulp exposure. These exposures are considered a valuable way for bacterial invasion to the pulp tissues. So, applying the restorative materials or cements by excessive pressure in deep cavities may contribute in pushing of bacteria to the pulp. 3|Page 3. The remaining dentin thickness: There are three cavity depth measurements that should be differentiated: a. The effective cavity depth refers to the distance along the dentinal tubule course between the pulp and the floor of the cavity. b. The clinical cavity depth refers to the depth of the cavity in dentin and enamel. c. The actual cavity depth refers to the thickness of the dentin bridge separating the pulp from the floor of the cavity. V- INFLUENCE OF CAVITY PREPARATION ON DENTIN-PULP ORGAN Definitions Cavity is the defect in the tooth due to caries process. Prepared Cavity is defined as, the mechanical alteration of the defective, injured or diseased tooth in order to receive a restorative material to restore esthetic and function. 1- Mechanical trauma: Pulpal irritation should be avoided during cavity preparation. Sharp hand cutting instruments are the most biologically cutting instruments because the energy applied is completely dissipated in the actual cutting. The rotary cutting instruments are also considered biologically accepted if used with proper cooling. The cut open dentinal tubules can transmit irritants from the oral cavity or restorative materials to pulp tissues (Cutting 1mm of dentine expose 30000 –45000 open dentinal tubules). To protect the pulp against traumatic injury the operator must avoid: a. Cutting across the recessional lines of the pulp. These lines represent the lines along which the pulp has receded during the growth of dentin. So, crossing these lines may cause pulp exposure. b. Over cutting of enamel to expose only minimum area of dentin. c. Over cutting of dentin and unnecessary deepening of cavity. d. Unnecessary application of pressure in scooping out carious dentin. 4|Page 2- Thermogenesis: Thermogenesis or heat generation is a highly damaging factor that affects the pulp vitality. Thermogenesis can occur due to: a. The friction between the cutting tool and tooth structure. b. Increased amount of pressure during cutting by rotary instruments. c. Increased rotational speed. d. Type of the tissues to be cut. e. Increased area of tool to tooth contact. f. Bur size and material: ✓ The larger the bur size, the greater the heat generation. ✓ Steel bur needs more pressure to have an effective cutting than the carbide bur of the same size. ✓ Diamond points generate more heat during cutting on either enamel or dentin & also their abrasive action leads to more heat generation. So the use of diamond points is confined to the enamel and superficial dentin. The heat generated during cavity preparation is dissipated through: 1) The tool. 2) The cut surface debris. 3) The surrounding atmosphere. 4) The tooth structure. Dentin is a good thermal insulator. It has a thermal tolerance zone between 85 and130°F. Beyond these extremes, its insulating capacity cannot be relied upon. The reaction of the tooth to thermal irritation depends on: 1. The proximity of the preparation to the pulp. 2. How the operative procedure is carried out. 3. The state of the pulp. Histologically, heat generation can cause:  Aspiration of the odontoblastic nuclei.  Irreversible pulpal damage (diminished reparative dentin formation)  Burn lesions in the pulp. 5|Page 3- Rotational speed of cutting instruments: Cutting at any rotational speed under dry condition causes pulpal changes, due to heat generation. Higher speed technique with sufficient coolant induces the least pulpal response. The pulpal response depends on: 1. The operating load. 2. Revolutions per minute of the cutting point. (RPM) 3. Diameter of the bur. 4. Temperature rise on the tooth surface. 5. Type of coolant used. 4- Pressure: Pressure is needed to induce efficient cutting especially in low speed equipment while at higher speed range, less pressure is required. Pressure is considered as the resultant of the force or load applied divided by the area of contact between the tooth and the tool, P = F/A. Pressure of the instrument on exposed dentin must be avoided as it causes: 1. Aspiration of the odontoblastic nuclei. 2. Pushing of some micro-organisms into the pulp leading to pulp inflammation. 5- Vibration: Vibration occurs due to eccentricity and run-out of rotary instrument. Bur eccentricity more than 0.025mm around its central axis. it causes:  Annoyance and discomfort to the patient  Cracks in enamel  Uncontrollable cutting  Different degrees of pulp damage such as: a. Rebound Response: appears microscopically as a limited area of necrosis at an area remote from the cut dentinal tubules. b. Calcio-traumatic Response: consists of several bands of hyper-calcified and hypo- calcified dark blue dentin indicating severe reduction in dentin formation & abnormality in the ratio of fibrils to cementing substance. 6|Page  c. Edema, fibrosis of the pulp tissues and reduction in the pre-dentin layer may also occur. 6- Dessication of Dentin Surface: Drying of dentin surface by a jet of air causes rapid outward movement of fluid through the dentinal tubules. According to the hydrodynamic theory of dentin sensitivity, this movement of fluid results in stimulation of sensory nerve of the pulp. Desiccation or dryness of dentin may occur due to: a. Overheating of dentin during cutting procedures. b. Using chemicals as alcohol to sterile the cavity. c. Using air as a coolant or in performing final toilet of the cavity. Therefore, efficient air-water spray should be used. 7- Remaining Dentin Thickness: Dentin permeability increases with increasing cavity depth. It affects the degree of pulpal injury resulting from restorative procedures and materials. A thickness of 2mm is required to protect the pulp from these effects. 8- Agents for Cavity Drying and Sterilization: Cavity sterilizing agents such as phenol and silver nitrate were used routinely which were highly irritating to pulp. Now, disinfectants such as chlorohexidine are used. 7|Page Recent cutting tools in cavity preparation: a. Laser: ✓ Lasers have been shown to effectively cut and ablate hard and soft tissues when the appropriate wavelength is selected. ✓ Water-laser (biolase) now is available. b. Carisolv: (chemo-mechanical caries removal): ✓ Carisolv is a chemical gel by which carious tooth structure can be painlessly removed. ✓ The advantage of Carisolv is patient comfort and the fact that no healthy tooth structure is removed. c. Micro abrasion and Macro abrasion: ✓ Micro/macro abrasion represents conservative alternatives for the reduction or elimination of superficial discolorations. ✓ These techniques are indicated only for stains or enamel defects that do not extend beyond a few tenths of a millimeter in depth. VI- INFLUENCE OF RESTORATIVE MATERIALS ON DENTIN-PULP ORGAN All restorative materials are tested for bio-compatibility before application. Factors responsible for pulpal injury: a. Toxic ingredients of materials b. Acidity (hydrogen ion concentration) c. Adsorption of water during setting d. Heat generation during setting e. Poor marginal adaptation resulting in bacterial contamination. 8|Page Biological perspective of the Restorative Material CEMENTS 1. Zinc oxide-Eugenol cement (ZnOE): ✓ It is biocompatible due to its eugenol content and good initial sealing and insulating ability. ✓ Eugenol has bacterio-static effect as it depresses cell respiration that stops bacterial growth. ✓ Eugenol has also palliative, sedative effect and provides thermal insulation for the pulp. 2. Zinc Phosphate Cement (Zn-Ph): ✓ Zinc phosphate cement has a thermal insulating property as that of enamel at the same time it is one of the most irritant cements due its phosphoric acid content. ✓ It has an initial low pH on setting & reaches neutrality after 48 hours. ✓ So, it is recommended to place a protective layer of a resin varnish under the cement because of its initial painful and damaging effects on the pulp. 3. Zinc Polycarboxylate Cement (P.C.C): ✓ It is a highly biologically acceptable material due to its polycarboxylic acid content which is an organic weak acid with low toxicity. ✓ The mixed cement pH is raised rapidly and reaches neutrality within 24hours. ✓ The large molecular size of the acid plus its ability to complex with proteins leads to limitation of acid diffusion through the dentinal tubules. 4. Glass Ionomer Cements (GIC): ✓ Glass ionomer cements are bio-compatible due to weak nature of its composition poly-acid. ✓ It releases fluoride which prevents bacterial ingress to underlying exposed dentin pulp system. ✓ Glass ionomer when used in class V cavity shows biocompatibility with the adjacent gingival tissues as the fluoride decreases bacterial adherence. 5. Resin cements: ✓ Dual-cure resin based cements are indicated for all ceramic crowns, ceramic veneers and porcelain inlays. ✓ Adequate light exposure must be ensured bucco-lingual and inter-proximally to ensure absence of residual monomer. 9|Page  LINER 1. Resin based liners: (Cavity Varnish): ✓ It is a copal resin that dissolves in an organic solvent (acetone). ✓ Biological merits of using the resin varnish under amalgam: a. Reduce micro leakage (under amalgam) b. Provides indirect protection to the pulp c. Prevents corrosion products or metallic ions from amalgam to pass into the dentinal tubules. d. It prevents acid penetration from the restoratives as zinc phosphate to pass into the exposed dentin and pulp. 2. Calcium hydroxide: ✓ Calcium hydroxide has minimal irritational quality to the pulp due to its alkaline pH (12,5pH) that neutralizes the acids of certain materials as zinc phosphate cement. ✓ It induces reparative dentin formation when applied on deep cavities or on cavities with vital exposures. ✓ Calcium hydroxide acts as a chemical barrier and prevents the escape of metallic ions from amalgam restoration into the dentinal tubules. ✓ It has also antibacterial potential. RESTORATIVE MATERIALS Amalgam: ✓ The response of the pulp to amalgam, in shallow cavities or deeper but lined, is minimal. ✓ The corrosive products released from amalgam may retard the reparative power of the pulp besides it diffuses into dentinal tubules causing tooth discoloration. ✓ Freshly placed amalgam is leaky, so it must lined by varnish. ✓ By aging the corrosive products will increase amalgam adaptability (self-sealing ability). ✓ As a metallic restoration thermal conductivity of amalgam in deep cavities should be considered, and excessive condensation force should be avoided in deep cavities. 10 | P a g e Resin composite: Restorative resin materials could be irritant to the pulp tissues due to: ✓ The chemical make-up of the materials. ✓ The leakage at the tooth-restoration interface. ✓ Old composites with severe marginal leakage caused by polymerization shrinkage. ✓ The visible light generators produce heat during composite curing. ✓ The effect of the acids used for etching purposes on the pulpal tissues depends on a number of factors: 1. The thickness of dentin between the restoration and the pulp. 2. The acid concentration. 3. Time of etching. 4. The technique sensitivity of the adhesive procedure. Cast Alloys: Irritation to the pulpo-dentinal organ comes through: 1) Its thermal conductivity. 2) The irritation effect of the cementing medium. 3) The pressure applied during impression making a mechanical irritation. BLEACHING AGENTS Bleaching agents used for non-vital and vital teeth for many years. These agents usually contain some form of peroxide (Hydrogen peroxide or carbamide peroxide). The cyto-toxicity depends to a large extent on the concentration of the Peroxide in the bleaching agent. Bleaching agents will also chemically burn the gingival tissues if not properly applied. So, gingival barrier must be applied before application of bleaching agents. 11 | P a g e VII- Gingival and Periodontal Irritation Gingival irritation is unavoidable especially whenever restorations are placed under the crest of the gingiva. Caries, operative procedures, and restorations may cause injury to gingiva and investing tissues. 1. Traumatic Instrumentation: Sub-gingival extension should be restricted to the removal of caries, or to place the cavity margin sub-gingivally for esthetic purposes. Traumatization of the epithelial attachment will lead to inflammation, congestion and blood stasis of gingival tissues causing: a. Blue or purple discolorations b. Gingival recession c. Successive bone loss d. Exposure of margins of the restorations 2. Matrices: The matrix band must be properly contoured and trimmed smooth to avoid impinging on the gingival. Wedges must be used to avoid gingival overhangs. 3. Overhangs of gingival margins of restorations: Causing gingival inflammation, congestion, permanent dark discoloration and recession as well as alveolar bone resorption and pocketing. 4. Rough surfaces of restorations: It results from poor finishing and polishing. Ragged edges of gingival cavity margins cause mechanical irritation to the gingival tissues and help retention of bacterial plaque to initiate chemical irritation. 12 | P a g e  5. Over-contouring: This result in: Too narrow embrasures Depriving the underlying gingiva from massage effect of food during chewing. Creation of a stagnation area gingival to the height of contour for the accumulation of bacterial plaque and subsequent caries recurrence. 6. Under- contouring: This creates: Too wide embrasures Exposure of the gingiva to direct pressure of the food during chewing cycles. Gingival and periodontal affections. 7. Open inter-proximal contacts: These factors cause food impaction and plaque retention leading to vulnerability to caries and periodontal involvement. Contacts should be verified for tightness by dental floss. Also, embrasures must be restored to the adjacent teeth and to the harmonious form. Open interproximal contacts are usually associated with insufficient separation to compensate the thickness of the matrix. Inter-dental separation is best accomplished using; the titanium rings in case of the sectional matrix system. Or, through the pre-wedging using large wooden wedge in cases of circumferential matrices. 8. Pulpo-periodontal communication: This may occur as a result of excessive depth pocketing and/or cervical extension of a neglected apical pathosis. D.Mai Mamdouh Head of Conservative and Restorative Dentistry Department 13 | P a g e